The identification of hippocampus-associated proteins and the isolation of cDNA molecules coding therefor is important in the field of neurophysiology. Thus, for example, such proteins are believed to be associated with memory functions and abnormalities in these proteins, including abnormal levels of expression and the formation of modified or mutated protein is considered to be associated with pathological conditions associated with memory impairment. The isolation of novel hippocampus-associated proteins and the associated DNA sequences coding therefor is consequently of considerable importance.
The present invention arose out of our investigation of hippocampus-associated proteins by differential screening of a rat hippocampus cDNA library. A cDNA species encoding a novel protein which we have designated Hct-1 was isolated and shown to be related to cytochromes of the P450 class.
The use of hybridization probes based on the rat Hct-1 sequence has led to the identification of homologues in other mammalian species, specifically mouse and human.
Cytochromes P450 are a diverse group of heme-containing mono-oxygenases (termed CYP's; see Nelson et al., DNA Cell Biol. (1993) 12, 1-51) that catalyse a variety of oxidative conversions, notably of steroids but also of fatty acids and xenobiotics. While CYP's are most abundantly expressed in the testis, ovary, placenta, adrenal and liver, it is becoming clear that the brain is a further site of CYP expression. Several CYP activities or mRNA's have been reported in the nervous system but these are predominantly of types metabolizing fatty acids and xenobiotics (subclasses CYP2C, 2D, 2E and 4). However, primary rat brain-derived glial cells have the capacity to synthesize pregnenolone and progesterone in vitro. Mellon and Deschepper, Brain Res. (1993), 629, 283-292(9) provided molecular evidence for the presence, in brain, of key steroidogenic enzymes CYP11A1 (scc) and CYP11B1 (11.beta.) but failed to detect CYP17 (c17) or CYP11B2 (AS). Although CYP21A1 (c21) activity is reported to be present in brain, authentic CYP21A1 transcripts were not detected in this tissue.
Interest in steroid metabolism in brain has been fuelled by the finding that adrenal- and brain-derived steroids (neurosteroids) can modulate cognitive function and synaptic plasticity. For instance, pregnenolone and steroids derived from it are reported to have memory enhancing effects in mice. However, the full spectrum of steroid metabolizing CYP's in brain and the biological roles of their metabolites in vivo has not been established.
To investigate such regulation of brain function our studies have focused on the hippocampus, a brain region important in learning and memory. Patients with lesions that include the hippocampus display pronounced deficits in the acquisition of new explicit memories while material encoded long prior to lesion can still be accessed normally. In rat, neurotoxic lesions to the hippocampus lead to a pronounced inability to learn a spatial navigation task, such as the water maze. The role of the hippocampus in learning has been further emphasized by the finding that hippocampal synapses, notably those in region CA1, display a particularly robust form of activity-dependent plasticity known as long term potentiation (LTP). This phenomenon satisfies some of the requirements for a molecular mechanism underlying memory processes--persistence, synapse-specificity and associativity. LTP is thought to be initiated by calcium influx through the NMDA (N-methyl D-aspartate) subclass of receptor activated by the excitatory neurotransmitter, L-glutamate, and occlusion of NMDA receptors in vivo with the competitive antagonist AP5 both blocks LTP and the acquisition of the spatial navigation task.
The induction of LTP is attenuated by simultaneous release of gamma-amino butyric acid (GABA) from inhibitory interneurons: activation of GABA.sub.A receptors antagonizes L-glutamate induced depolarization of the postsynaptic neuron and interplay between the GABA and L-glutamate receptor pathways is thought to modulate the establishment of LTP. Interplay between these two circuits is emphasised by the finding that some aesthetics (e.g. ketamine) act as antagonists of the NMDA receptor while others, such as the steroid aesthetic alfaxolone, are thought to be agonists of the GABA.sub.A receptor. It is of particular note that some naturally occurring steroids, such as pregnenolone sulfate, act as agonists of the GABA.sub.A receptor, while pregnenolone sulfate is also reported to increase NMDA currents. Although neurosteroids principally appear to exert their effects via the GABA.sub.A and NMDA receptors, there have been indications that neurosteroids may also interact with sigma and progesterone receptors.
Despite considerable interest in the action of neuro-active steroids, and possible roles in modulating synaptic plasticity and brain function, little is known of pathways of steroid metabolism in the central nervous system. As part of a study into the molecular biology of the hippocampal formation, and the mechanisms underlying synaptic plasticity, we have sought molecular clones corresponding to mRNA's expressed selectively in the formation. One such cDNA, Hct-1 (for hippocampal transcript), was isolated from a cDNA library prepared from adult rat hippocampus. Sequence analysis has revealed that Hct-1 is a novel cytochrome P450 most closely related to cholesterol- and steroid-metabolizing CYP's but, unlike other CYP's, is predominantly expressed in brain. The present invention provides molecular characterization of Hct-1 coding sequences from rat, mouse and humans, their expression patterns, and discusses the possible role of Hct-1 in steroid metabolism in the central nervous system.
DNA sequences encoding hitherto unknown cytochrome P450 proteins have now been identified and form one aspect of the present invention.